7-Hydroxymitragynine, an indole, is an active alkaloid in the plant Mitragyna speciosa, commonly known as Kratom. It has opioid agonistic activity.[1] "The potency, calculated using pD (2) values, was 30- and 17-fold higher than that of mitragynine and morphine, respectively. Antagonism of naloxone on concentration-response curves for 7-hydroxymitragynine confirmed its opioid effect. These results suggest that the opioid effect of M. speciosa is mostly based on the activity of 7-hydroxymitragynine."[2]